UPDATE: MASS EFFECT 3 Multiplayer Details Released!

UPDATE 10/11/11: Overnight on the official forums of Mass Effect 3, BioWare's Community Coordinator Chris Priestly released full details on the new multiplayer mode for the game.

The game will have a new, completely optional cooperative multiplayer mode as part of the "Galaxy At War" section of the game. Gamers who want a 100% "Galactic Readiness" can achieve that solely through the standard single-player gameplay, or can gain points toward that via the new 4-player team-up.
For the co-op mode, gamers will customize characters of various races like "Turians, Krogans, Asari, and more..." says Priestly. Here, you form a special operations squad and go on missions together to better prepare for the war against the Reapers. Player characters will be able to upgrade weaponry and level-up, increasing their efficiency as they go.
It's heavily implied by Priestly that multiplayer is not the only component of the "Galaxy at War" system. Details of the other parts are promised as on the way over the coming months. The key takeaways from Priestly's post are clearly that multiplayer is fully optional and will not interfere with the single player game, that multiplayer was not the source of the date being pushed back (in fact, it's being developed by their new Montreal studio), and that this is meant purely to enhance the overall experience. More on Mass Effect 3 multiplayer from New York Comic Con this weekend.
Original Story: No more rumors: Mass Effect 3 has multiplayer. That's what two magazine covers released today show, with both due to hit the stands next week.
Australian magazine PC PowerPlay and US-based Official Xbox Magazine will both be carrying the details first, though presumably we'll see details in most other major gaming magazines soon after. Neither cover blurb reveals much, though OXM does indicate the multiplayer game mode will have something to do with the single-player campaign.
The first two Mass Effect titles have been highly-acclaimed best-sellers, and Mass Effect 3 was named by an analyst this summer as the most anticipated game in the business. It was also pushed back to a March 6, 2012 release, presumably so features like multiplayer could be integrated and perfected.
Executive Producer Casey D. Hudson took to twitter to give one new hint as well, indicating the multiplayer will be in the form of cooperative missions:
"Yes, co-op MP missions for #ME3: they're real, and they're spectacular. Rest assured it's nothing of what you've feared. More soon..."
BioWare has made their name in single-player role playing games like Knights of the Old Republic and the Dragon Age series, but are also currently working on Star Wars: The Old Republic, a massively multiplayer online role playing game.
Stay tuned for details on Mass Effect 3 multiplayer later this week!
Related Stories: